1. Field of the Invention
The present invention relates to a rear wheel steering system for an automotive vehicle.
2. Description of the Prior Art
A rear wheel steering system for an automotive vehicle is to steer the rear wheels in accordance with the front wheel steering, for the purpose of allowing a small turn and improving the turning stability of the vehicle. Various kinds of rear wheel steering systems have been conventionally proposed and disclosed, for example, in Japanese Patent Application Laid-open Publication No. 63-287,676. Here, the rear wheel steering angle .delta..sub.r is determined in accordance with the following equation: ##EQU1## where
.delta..sub.f : Front wheel steering angle
T: first order delay time constant of rear wheel steering system
K: proportional constant
.tau.: first order differential constant
.tau.': second order differential constant
S: complex variable
In the known control of the rear wheel steering angle described above, the proportional constant K determines a same-phase component of the rear wheel steering in the same direction as as the front wheel steering, which is a proportional component contributing to improve the turning stability in a steady state, while the differential constants .tau. and .tau.' determine an opposite-phase component of the rear wheel steering in the opposite direction to the front wheel steering, which, in turn, is a differential component contributing to improve the transient turning response characteristic.
In this case, the rear wheel steering control constants K, .tau. and .tau.' are respectively determined only as the function of a vehicle speed V which achieves a front and rear wheel steering angle ratio .delta..sub.r /.delta..sub.f shown, for example, in FIG. 12, and are fixed with reference to the front wheel steering angle. Therefore, despite the fact that a large front wheel steering angle is an indication of requirement of a large turning amount of the vehicle, when the rear wheel steering is performed regardless of the front wheel large steering condition, the proportional component determining the same-phase steering amount of the rear wheel tends to inhibit the turning property of the vehicle, thereby making a desired steering operation difficult contrary to the driver's demand.
It is widely known, particularly in the case of a front wheel drive vehicle, that the vehicle exhibits an understeering tendency when it is turned round while being simultaneously accelerated, due to a reduced ground contact area of the front wheel tires as a result of decrease in the load applied thereto, and also due to a relatively small side force generated during the turning of the vehicle by the front wheel tires which are applied with driving force.
The rear wheel steering technology described above is essentially to steer the rear wheels in the same phase as the front wheel steering, with an increased same-phase component based on the proportional component, though the same-phase rear wheel steering enhances the understeering tendency upon simultaneous turning and acceleration of a front wheel drive vehicle. This means that, when a front wheel drive vehicle is accelerated and turned round under a front wheel large steering condition, the resulting enhanced understeering tendency makes it extremely difficult for a driver to properly steer the vehicle as intended.